The present invention relates to a method for protecting printed data, as well as a machine implementing such a method.
A heat-transfer printer, and more particularly a heat-transfer plastic-card printer, comprises:                a feed system that makes it possible to supply the printer with a blank plastic card, this feed system may be a card-by-card feed system or a reservoir of a plurality of cards provided with a card-separation system,        an ejection system that makes it possible to eject each plastic card from the printer after printing, and        between the feed system and the ejection system, a print module for printing on the plastic card.        
The print module comprises:                a feed roller on which a transfer film is wound, carrying an area of ink and an area of varnish to be deposited,        a recovery roller on which the transfer film is wound after the ink and varnish have been applied to the plastic card,        a print head between the feed roller and the recovery roller, and        a backing roller disposed against the print head.        
The transfer film is positioned between the plastic card and the print head.
When a plastic card is to be printed and varnished, the transfer film and the plastic card to be printed are synchronised with respect to the print head so that said plastic card and a blank area of the transfer film are presented simultaneously at the print head. The synchronisation takes place for example by means of position detectors which detect among other things the front edge of the plastic card when the plastic card advances towards the print head.
During printing, the print head heats the transfer film according to the characters to be printed, which causes the transfer of the ink from the transfer film onto the plastic card. As printing progresses, the plastic card and the transfer film advance simultaneously under the print head in order to print the plastic card, and the transfer film winds progressively on the recovery roller.
Following the printing of the characters on the card, the printer is designed to apply the varnish to the card in order to protect the printed elements from external attacks.
To deposit the varnish, the transfer film and the card are repositioned upstream of the print head and once again synchronised with respect to the print head, and then the heating of the print head and the simultaneous advance of the plastic card and transfer film cause the transfer of the varnish onto the plastic card.
Because of the presence of the ink on the plastic card, the characters printed on the plastic card are in relief and, on passage of the varnish area, these characters in relief deform the transfer film at the varnish area.
By recovering the transfer film wound on the recovery roller, it is then possible to find the data that were printed by observing the varnish area, which is unsatisfactory from the confidentiality point of view.